This invention relates to apparatus for determining the proportion of particles of less than a predetermined size in a fluid suspension.
In many industries it is required to measure the particle size parameters of particulate solid materials and, in the research and quality control laboratories of such industries, it is often necessary to obtain information quickly and accurately about the particle size distribution of a large number of samples. Information concerning the proportion of particles having a linear dimension greater than about 50 microns is generally most conveniently obtained by the use of test sieves of different aperture sizes. However, when it is required to know the percentage by weight of particles smaller than about 50 microns, it is generally necessary to use a method based on gravitational or centrifugal sedimentation. By Stokes' Law, the particles suspended in a fluid, assuming that they are deflocculated, i.e., are present as discrete particles and not as agglomerates containing particles of different sizes, in the absence of turbulence each fall through the suspension at a velocity which reaches a constant value dependent on the square of the particle radius (assuming that the particle is spherical), the difference between the density of the particle and the density of the fluid and the viscosity of the fluid. When particles are not spherical a true determination of exact particle size is not possible by this method, but the results which are obtained in terms of "equivalent spherical diameter" are nevertheless very useful in comparing the particle size distributions of material having similar particle shape.
It is known to determine the proportion of particles in a suspension of less than a predetermined size by thoroughly mixing the suspension, withdrawing a sample from a particular level within the suspension by means of a pipette, evaporating the sample to dryness, and weighing the resultant dry solid material; and then allowing the suspension to settle under quiescent conditions for the time which according to Stokes' Law is required for particles of the predetermined size to fall from the surface of the suspension to the particular level, withdrawing a second sample from that level, and evaporating and weighing that second sample, the weight obtained in the second step being expressed as a percentage of the weight obtained in the first step to give the required proportion. However such a procedure is time consuming and requires considerable skill if accurate and repeatable results are to be obtained.